Considerable interest has been focused on the reduction of regulated exhaust constituents from internal combustion engines. Recently, focus has been on engines that emit high levels of exhaust particulates with particular attention paid to diesel engines. Diesel engine exhaust is a heterogeneous mixture containing not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOX”), but also condensed phase materials (liquids and solids) which constitute particulate matter. Catalyst compositions, and substrates on which the catalysts are disposed, may be provided in diesel engine exhaust systems to convert certain, or all of these exhaust constituents to non-regulated components. For example, diesel exhaust systems may include one or more of a diesel oxidation catalyst, a diesel particulate filter and a catalyst for the reduction of NOx.
One after treatment technology in use for particulate matter reduction is the particulate exhaust filter commonly referred to as a diesel particulate filter (“DPF”). There are several known filter structures that are effective in removing the particulate matter from engine exhaust such as honeycomb, wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. The ceramic wall flow monoliths have experienced significant acceptance in high particulate automotive exhaust applications. The filters are structures for physically removing particulate matter from the exhaust and, as such, accumulating particulates relies on the continuing integrity of the filter media. DPF's, especially those constructed of ceramic material may be brittle and can form cracks or other types of failure during normal operation, or during regeneration cycles when the accumulated soot is intentionally combusted at high temperatures to clean the filter and manage exhaust back pressure. Current legislation demands on-board detection of a failed DPF, defined as 90 mg of soot passing through the filter between regenerations. This standard will be raised in the next regulatory tier to 40 mg of soot with the final regulatory tier requiring detection of 17.5 mg of soot between regenerations.
A leading method for on-board detection of a failed DPF relies on the use of individual pressure sensors positioned both upstream and downstream of the filter. Under certain conditions of exhaust flow and soot content in the DPF, the pressure drop across an uncompromised filter has an assumed value. When the filter is compromised from a crack or other failure (ex. melting), the pressure drop will theoretically change thereby providing an indirect method for determining that soot is passing through the system rather than being filtered out. The method does not directly measure the quantity of soot passing through the filter but rather infers its presence in the DPF based on a pressure measurement. Another drawback to this system is that detection can only be performed when certain exhaust flow and soot level conditions in the DPF are met, usually immediately after a regeneration event when no soot is in the DPF. As a result the method does not provide for continuous monitoring of the integrity of the filtration system. Even under ideal conditions, the sensitivity of the system is limited due to the small pressure differential caused by a crack in the DPF. Accordingly it is desirable to provide a system and method of continuous measurement, independent of conditions, having an improved level of robustness over the current method.